1. Field of the Invention
The present invention relates to a multi-stage multi-pole electrode arrangement used for an electrostatic lens system which converges a beam of electric charges and is applied, for example, to a corpuscular ray accelerator, mass spectrometer and ion beam analyzer.
2. Description of Related Art
As disclosed in JP-A-59-215650 (1984) which corresponds to U.S. Pat. No. 4,553,029, in a double convergence mass spectrometer an electrostatic lens unit of quadrupole electrode system is widely used as a measure for converging charged particles or ion beam and when no sufficient convergence is obtained with such electrostatic lens unit of a single stage, a plurality of such electrostatic lens units are used and arranged in two or three stages in series to obtain a required convergence.
FIG. 4 is a perspective view showing one example of conventional electrostatic lens arrangements of muliti-stage multi-pole electrode system.
The illustrated conventional electrostatic lens arrangement relates a three stage quadrupole electrode system wherein a first stage quadrupole lens unit 1A, a second stage quadrupole lens unit 1B and a third stage quadrupole lens unit 1C, each of which is constructed independently, are accomodated in series within a cylindrical casing 2.
In each of the electrostatic lens units of multi-pole electrode system for the respective stages four rod shaped electrodes 3, wherein four electrodes 3A for the first stage, four electrodes 3B for the second stage and four electrodes 3C for the third stage, which are generally called as electrodes 3, are arranged in parallel around a circumference surrounding a passage of the charged beam with an equal interval of 90.degree. and rods 4' for supporting the electrodes 3 are inserted through the respective electrodes 3 in the axial direction at the center thereof. At both axial ends of the respective quadrupole electrodes disks 5-1 and 5-2 are disposed opposing each other and both ends of the supporting rods 4' for the respective electrodes 3 are secured to the respective disks 5-1 and 5-2, and further the respective disks 5-1 and 5-2 are provided with a beam passage aperture 6.
Further, conventional electrostatic lens arrangements of multi-pole electrode system are also disclosed in JP(PCT)-A-59-500642 (1984) which corresponds to WO-A-83/03498, JP-A-2-257558 (1990) and JP-A-2-270256 (1990).
The conventional electrostatic lens arrangements of multi-stage multi-pole electrode system as explained above are constituted by combining a plurality of independent electrostatic lens units of multi-pole electrode system one by one in series, therefore such conventional electrostatic lens arrangements had the following drawbacks to be improved.
(1) When constituting an electrostatic lens arrangement of multi-stage it is necessary to assemble the multi-pole electrodes for the respective stages in such a manner that the center axes of the corresponding electrodes for the respective stages coincide each other and align straight lines in order to eliminate offsetting of axes of the respective electrostatic lens units, however the supporting rods for the electrodes for the respective electrostatic lens units are independent, it is difficult to coincide the axes of the corresponding electrodes for the respective stages with a high accuracy. When there arises an error in axis coincidence with the corresponding electrodes an aberration due to axis offsetting in electrostatic lens units with respect to the charged beam is generated and a convergence estimated according to a theory can not be achieved.
(2) Further, an electrostatic lens arrangement of multi-pole electrode system for n stages requires n time larger number of parts than that required for a single stage, therefore the time for assembling the same as well as the cost therefor increase.